Hindawi Publishing CorporationLaser ChemistryVolume 2006, Article ID 75831, 7 pagesdoi:10.1155/2006/75831

Research ArticleLaser Cleaning Tests on Archaeological Copper AlloysUsing an ND:YAG Laser

Capucine Korenberg and Alexandra Baldwin

Department of Conservation, Documentation and Science, The British Museum, Great Russell Street,London WC1B 3DG, UK

Received 14 September 2006; Revised 3 November 2006; Accepted 6 November 2006

Recommended by Costas Fotakis

Laser cleaning tests were performed on five archaeological copper alloy objects using a Q-switched Nd:YAG laser at 1064 nm. As acomparison, a section of each object was cleaned mechanically. Prior to cleaning, cross-sections were prepared to characterise thecorrosion crust and help to locate the position of the original surface. Laser cleaning was not successful at removing burial depositson two of the objects. For the other three objects, the laser removed most of the corrosion crust. This was not always satisfactory, ascleaning was sometimes accompanied by the loss of the original surface. In addition, laser-cleaned surfaces were matt compared tomechanically cleaned surfaces. In some instances, the former had a disfiguring purple hue which was attributed to the formationof particles that could be seen when examining the surface using scanning electron microscopy. For all the objects examined here,superior results were obtained by mechanical cleaning.

Copyright © 2006 C. Korenberg and A. Baldwin. This is an open access article distributed under the Creative CommonsAttribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work isproperly cited.

1. INTRODUCTION

The British Museum holds many archaeological metal arte-facts within its collections. Most metal finds are covered bycorrosion crusts that may have to be removed and the presentstudy is focused on the cleaning of archaeological copper al-loys. The formation of corrosion products on archaeologi-cal copper alloys can be complex and differ according to theburial environment and composition of the metal [1–4]. Forexample, in the case of bronzes, a selective copper dissolu-tion often takes place leaving the outer corrosion layers of ar-chaeological bronzes enriched in tin [5, 6] and brasses withsubstantial amounts of zinc are known to lose zinc throughdezincification [2]. A detailed review of the corrosion pro-cess in copper alloys is beyond the scope of the present paperand only a schematic description of the process for pure cop-per is presented here. Throughout the lifetime of an object,the surface oxidises producing a very thin compact corrosionlayer. During burial, the copper under the surface is furtherattacked and intergranular corrosion occurs. A compact layerof cuprite, Cu2O, forms along the grain boundaries, fillingthe voids. This layer is called the “primary” cuprite layer. Ascorrosion progresses, the metal core reduces in size becomingpitted and formless. This is accompanied by the migration

of copper ions through the primary cuprite layer and theformation of secondary corrosion products such as cuprite,malachite, and basic copper chlorides [1, 2]. The outer-mostlayers of corrosion often incorporate minerals and quartzgrains from burial deposits [2]. Variation in burial environ-ments will produce additional areas of different compounds,including sulphides and chlorides.

In this context, cleaning is defined as the beneficial re-moval of altered material originating from the object or ex-ternally deposited material. Archaeological metals are usu-ally not cleaned back to the metal surface as, in the majorityof cases, the metal core will be shapeless. Instead, the “orig-inal surface” is sought. In conservation, the term originalsurface denotes a layer within the corrosion products wheredecoration, tool marks, or evidence of wear can be found.The position of the original surface in copper alloy corrosioncrusts varies depending on the condition and state of miner-alisation of the object and can be anywhere in the corrosioncrust. Detection of the original surface in the layers of corro-sion can be difficult, but it is often a more or less continuouscompact layer, which differentiates it from the less dense sec-ondary corrosion products. The original surface retained inthe mineralised deposits often has a smooth, even, and lus-trous surface from original finishing of the object or due to

2 Laser Chemistry

polishing throughout its useful life. Conservation cleaningdoes not aim to remove every trace of corrosion overlyingthe surface, but to reveal and preserve the original surface.Overcleaning can lead to removal of intergranular corrosionor disruption of the original surface creating a matt appear-ance. The choice of cleaning method for a particular objecthas to be assessed by conservators in terms of risk and bene-fit to the object, as no method currently in use is 100% suc-cessful. Chemical cleaning methods are not easily controlledand can be unpredictable, usually resulting in the loss of theoriginal surface. Air abrasive, whilst useful, is often difficultto control and can produce a matt surface. The most accu-rate and adaptable method for archaeological copper alloysis mechanical cleaning using a scalpel. However, mechanicalcleaning can take a long time and damage can occur eitherthrough breakage due to pressure applied to the object orscratching of the surface.

2. EXPERIMENT

Laser cleaning has gained considerable success as a valuablemethod of conservation since the 1970s and often has the ad-vantage of being faster than conventional techniques, whileproducing equal or superior results on certain materials (e.g.,[7]). Recently investigations of the use of the Nd:YAG laser toremove corrosion crusts on archaeological copper alloy arte-facts have taken place. The results of these investigations havebeen varied. Pini et al. [8] laser-cleaned 16 bronze objectscovered by green calcareous accretions and silicates, preserv-ing the oxidation layer beneath. The Nd:YAG lasers used intheir study had a pulse duration of either 20 µs or 2–10 ns,and Pini et al. report that long pulses were more efficient atremoving thick encrustations than short pulses. They some-times observed discolourations on the surface of calcare-ous encrustations, which changed from green to grey or red.This was not considered a problem since all the objects werecleaned down to the cuprite layer, where no colour changeever took place. A comparison was done with mechanicalcleaning on one of the objects and the authors report thatmanual cleaning was more time-consuming and did not pro-duce the same homogeneity of cleanliness as laser cleaning.In another study, Batishche et al. [9] have performed suc-cessful removal of corrosion layers from a bronze fastener toreveal surface detail and decoration using a combination ofND:YAG lasers with pulse durations of either 100–120 µs or15 ns. They did not report any side effects from laser clean-ing. Drakaki et al. [10] have laser-cleaned two Roman coinsusing a Q-switched Nd:YAG laser operating at 532 nm and0.4 J/cm2. On the first coin, the surface obtained was rathersmooth with a bright colour and, from the photograph pub-lished, surface detail was lost. For the other coin, the laser-cleaned surface was smooth but had a dark colour, whichwas deemed less satisfactory. Although these three studiesindicate that laser cleaning is successful at removing corro-sion layers, none of them clearly assessed where the originalsurface of the objects lay and whether it was preserved bylaser cleaning. The original surface does not always lie at theprimary cuprite layer and it has not been demonstrated thatlaser cleaning is successful at retaining surfaces above this.

Object 1 Object 2

Object 3 Object 4 Object 5

Cm

Figure 1: Objects after cleaning: the left side of each object has beenlaser-cleaned and the top right has been mechanically cleaned. Thebottom right side was left uncleaned.

Descriptions of success or failure can be subjective dependingon expectation and cleaning requirement. Systematic com-parison with mechanical methods is also required so that theresults can be objectively assessed before application.

The British Museum has recently acquired an Nd:YAGlaser and the present study forms part of ongoing researchinto its use in cleaning metal artefacts in the Museum’s col-lection. In particular, the aims of the current work were tolook in more detail at the effects of laser on archaeologicalcopper alloy in relation to its practical use in conservation,and add to the body of work in this field. The use of thelaser has interesting possibilities as an alternative low con-tact cleaning method and the parameters of the laser requireassessment before application on registered museum objects.Laser cleaning tests were performed on several archaeologicalcopper alloy objects using an Nd:YAG laser and whether thelaser treatment preserved the original surface was carefullyassessed. In addition, mechanical cleaning was conducted asa comparison.

3. MATERIALS AND METHODS

3.1. Samples

As it is not possible to produce in the laboratory corrosioncrusts similar to those present on archaeological objects, testswere performed on five unregistered archaeological copperalloy objects of unknown provenance (see Figure 1). Exceptfor Object 1, the corrosion crust on each object was rela-tively thin and appeared homogeneous across the surface ofthe object. All the objects were covered by silicaceous burialdeposits.

Prior to cleaning, analyses were performed to deter-mine the composition of the alloys and the corrosion lay-ers. Sections of each object were removed using an Isomet

C. Korenberg and A. Baldwin 3

saw, embedded in an epoxy resin, ground on carborun-dum paper and polished with diamond pastes of 6 µm and1 µm. X-ray fluorescence (XRF) analyses were carried out onthe uncorroded metal core of the objects using an ArtTAXspectrometer (voltage 50 kV and current 0.80 mA) and theresults are shown in Table 1. The corrosion layers were ex-amined in cross-section using polarised light microscopy,Raman spectroscopy, scanning electron microscopy (SEM),and energy dispersive X-ray analysis (EDX) to help in locat-ing the position of the original surface. It should be notedthat oxygen is easily detected using EDX and all the cor-rosion crusts were found to contain significant amounts ofoxygen. However, the amount of oxygen calculated from theEDX analysis cannot be considered reliable due to inherentdifficulties in calculating the background correction. There-fore, oxygen was not included in the calculations of the rel-ative amounts of present elements. Also, it was not alwayspossible to obtain Raman spectra of the compounds presentin the corrosion crusts although most corrosion productsare Raman active. This could be because corrosion prod-ucts on archaeological metals are often poorly crystallised[6]. The results of the analyses are reviewed for each objectin turn.

Object 1

Analysis of Object 1 indicated that this is a leaded tin bronze.The corrosion crust on this object was uneven, with areascovered by either a thin corrosion crust or warts. The sectionwas taken on an area covered by a thin corrosion crust. Nextto the metal core was a very thin layer of oxidised metal and,beyond this, a thick corrosion layer containing mostly copperand silicon with small amounts of iron and aluminium. TheRaman spectrum obtained for this layer suggested the pres-ence of chrysocolla, (Cu, Al)2H2Si2O5(OH)4 · xH2O. Theoriginal surface appeared to be just above the metal oxidelayer.

Object 2

Object 2 is a leaded brass. There was a layer of cuprite nextto the metal core, followed by three homogeneous layers.The layer immediately above the cuprite contained mostlycopper, but also silicon and traces of zinc. The second layerhad a similar composition as the first layer, but containedslightly less silicon and also small amounts of tin and iron.The outer layer contained mostly copper and silicon, withsmall amounts of iron and zinc. Chrysocolla was possiblypresent in this layer. The original surface appeared to lie justabove the cuprite layer.

Object 3

Object 3 is a bronze containing copper, lead, tin, and zinc.Adjacent to the metal core is a layer of cuprite and possiblycassiterite. As distinct from the other objects, Object 3 has acompact layer of cerrusite which is not homogeneous acrossthe surface of the object but interrupted by discrete areas of

Table 1: Composition of uncorroded metal core determined usingXRF. (The results should have an accuracy of c. ± 1-2% for copperand c.± 5–10% (relative) for tin, zinc, and lead when present above10% in a copper alloy, deteriorating to c.± 20–30% (relative) whenpresent above 1% but below 10%.)

Object Composition (% wt)

1 86% Cu, 13% Sn, 1% Pb

2 77% Cu, 20% Zn, 3% Pb

3 87% Cu, 5% Pb, 5% Sn, 2% Zn, traces

4 73% Cu, 20% Zn, 7% Pb

5 78% Cu, 21% Zn, 1% Sn

Metal Cuprite Cerrusite Burial deposits

� 200 µm

Figure 2: SEM picture of the cross-section of Object 3, showing thecorrosion layers before cleaning. The core metal and the primarycuprite layer are located on the left of the picture. The light greylayer in the middle of the section is cerrusite and to its right is acorrosion layer containing particles of cerrusite and burial deposits.The original surface is at the top of the cerrusite layer. (Length ofscale marker is 200 µm.)

cuprite and malachite. The presence of cerrusite has been re-ported on archaeological bronze artefacts rich in lead (e.g.,[11]). The outermost layer of corrosion was complex con-taining large amounts of lead, oxygen, and copper, as well asparticles of cerrusite and burial deposits. The original surfacelies at the top of the compact cerrusite layer, see Figure 2.

Object 4

The metal core of Object 4 is leaded brass. The corrosionlayer next to the core contained cuprite. Above the cupritewas a layer containing copper, silicon, phosphorous, andlead. Raman spectroscopy showed the presence of malachiteand possibly chrysocolla. The outermost layer contained in-clusions of silica, copper, iron, lead, and phosphorous andis probably corrosion mixed with deposits from burial. Theoriginal surface of the object was at the top of the malachiterich layer.

4 Laser Chemistry

Table 2: Parameters used for the laser cleaning.

Object Fluence Condition

1 400 mJ/cm2 No water

2 400 mJ/cm2 Immersed in water prior to cleaning

3 240 mJ/cm2 Immersed in 50/50 water/IMS∗

prior to cleaning

4 900 mJ/cm2 No water

5 600 mJ/cm2 Water applied on surface

∗IMS= industrial methylated spirits.

Object 5

Object 5 is a brass containing copper, zinc, and tin. Next tothe core lies a very thin layer of cuprite with traces of zinc andtin. The adjacent corrosion layer contained a large amount ofcopper, silicon, tin, and phosphorous. It was found to con-tain malachite and possibly chrysocolla. The original surfaceof the object appears to be contained in this layer.

3.2. Methods

For the cleaning tests, the top surface of each object was di-vided into three areas: half of the surface was treated by laser,a quarter was left untreated, and the remaining quarter wascleaned mechanically. The laser used was a Lynton PhoenixAthena laser (wavelengths: 532 and 1064 nm, pulse dura-tion: 5–10 ns). Only the 1064 nm wavelength was used in thepresent work as preliminary tests at 532 nm indicated thatthe cleaning process was extremely slow, making the use oflaser at this wavelength impractical. The fluence used variesconsiderably, as each laser cleaning test was performed at theminimum fluence at which the overlying soil deposits couldbe removed on each sample object. Once the working en-ergy levels had been established, the laser spot area was es-timated by taking a burn pattern on a photographic paperand the average fluence was calculated by dividing the energyper pulse by the laser spot area. To achieve an even clean, thepulses were overlapped slightly, therefore only a few pulsesper area were applied to the surface. A liquid was applied tothe surface of some of the objects while using the laser, asthis has been reported to improve the laser cleaning of met-als (see, e.g., [10, 12–14]). Some objects were immersed in aliquid prior to testing to increase penetration into the corro-sion crust. The parameters used for the laser cleaning of eachobject are summarised in Table 2. Mechanical cleaning wasperformed with a scalpel fitted with a Swann-Morton No.15stainless-steel blade under×40 magnification. All the objectswere more time consuming to clean mechanically comparedto the laser treatment.

After the cleaning tests, the treated surfaces were exam-ined using an optical microscope (up to ×60 magnification).The corrosion crusts and the cleaned surfaces were examinedusing SEM-EDX and Raman spectroscopy. EDX spectra weremeasured using a JEOL JSM840 scanning electron micro-scope with an Oxford Instruments ISIS EDX analyser. Ramanspectroscopy was carried out using a Jobin Yvon LabRam

Infinity spectrometer with a green laser with a wavelength of532 nm. Spectra were measured in the 100–1600 cm−1 regionwith a resolution of 2 cm−1. Each spectrum was collected forbetween 10 and 60 seconds and at least five repetitions wereused to produce a spectrum.

4. RESULTS AND DISCUSSION

4.1. Results of cleaning tests

Where the laser cleaning had been successful at removingburial deposits, the laser-cleaned surface was analysed usingSEM-EDX and Raman spectroscopy (see Table 3 for the re-sults of the SEM-EDX analyses). The results are reviewed foreach object in turn.

Object 1

It could be observed with the naked eye that, unlike mechan-ical cleaning, the laser treatment had not removed the burialdeposits from Object 1 completely. Due to the uneven thick-ness of the corrosion crust on this object, it was not possibleto remove the warts during the laser cleaning tests withoutexposing the core metal in adjacent areas covered by thinnercorrosion layers. The dendrite structure of the metal was alsoexposed as the laser had removed the intergranular corro-sion. Slight discolouration to the cuprite layer was also notedunder high magnification. As no liquid was used during lasercleaning on Object 1, it was investigated whether using aliquid would improve the cleaning process. Laser cleaningtests were done on the back of the object using water, butit was still not possible to remove the burial deposits. This isin agreement with the work of Degrigny et al. [15] who re-ported that using a liquid did not have any effect when lasercleaning tarnished silver threads.

Object 2

When the cross-section of Object 2 was examined usingSEM-EDX prior to cleaning, it was found that the metal corewas covered by several layers of corrosion, with the outer-most layers containing small amounts of iron. It appearedthat the original surface lies just above the primary cupritelayer. The laser treatment removed the burial deposits fromObject 2, but the laser-cleaned surface had a matt appear-ance, which was very different from the lustrous surface ob-tained by mechanical cleaning, suggesting that the originalsurface had been removed. This can be seen in Figure 3 whereoriginal tool marks on the surface are less visible in the laser-cleaned areas. In addition, it was observed using an opticalmicroscope that the laser had uncovered the core metal in afew very small areas (approximately 1 mm in size). This couldbe due to the hot spots in the laser beam or heterogeneitiesin the corrosion products. When analysing the laser-cleanedsurface using SEM-EDX, the copper content was high andno iron was detected (see Table 3). In contrast, the presenceof iron on the mechanically cleaned surface and relatively lowcopper content suggest that not all the outer-most corrosion

C. Korenberg and A. Baldwin 5

Table 3: SEM-EDX analyses for the mechanically cleaned surface and laser-cleaned surface for the objects, on which laser cleaning wassuccessful at removing burial deposits.

Object Mechanical cleaning (% wt) Laser cleaning (% wt)

2 53% Cu, 19% Si, 16% Zn, 5% Pb, 3% Fe 88% Cu, 3% Si, 4% Zn, 5% Pb

3 7% Cu, 93% Pb 52% Cu, 38% Pb, 10% Sn

5 64% Cu, 28% Si, 3% Ca, 3% Fe, 2% P 92% Cu, 2% Si, 6% Zn

(a)

(b)

Figure 3: Cleaning results for Object 2: (a) laser-cleaned sur-face, (b) mechanically cleaned surface. (Horizontal field of view is9 mm.)

layers have been removed. This indicates that, unlike me-chanical cleaning, the laser has removed the outer-most cor-rosion layers and possibly some of the original surface. Someareas cleaned by the laser had become very dark compared tothe mechanically cleaned surface, while others had a slightlypurple hue. This is illustrated in Figure 3. Purple/blue tingeson oxidised copper plates irradiated by a Q-switched Nd:YAGlaser have been reported by Kearns et al. [16]. Peaks at 301and 624 cm−1 in the Raman spectrum on some areas of thelaser-cleaned surface suggest the presence of tenorite, CuO.Tenorite forms at temperatures between 400 and 600◦C andlaser irradiation has been reported to cause the oxidation ofcuprite to tenorite [16].

Object 3

The corrosion crust on Object 3 was different from that onthe other objects as it contained cerrusite. When analysing

the cleaned surfaces, the amount of lead was much lower onthe laser-cleaned surface than on the mechanically cleanedsurface (see Table 3). In addition, cuprite and cerrusite weredetected by Raman spectroscopy on most of the laser-cleanedsurface, whereas only cerrusite was detected on the mechan-ically cleaned surface. This indicates that, although a thinlayer of cerrusite has been left by the laser treatment, mostof the compact cerrusite layer, and therefore the original sur-face, have been removed. Also, the laser-cleaned surface wasmatt compared to the lustrous surface uncovered by mechan-ical cleaning further suggesting overcleaning and damage tothe surface.

Object 4

The malachite rich layer on this object should be retainedduring cleaning as it is where the original surface lies. Dur-ing mechanical cleaning, it was observed that the fragile orig-inal surface had been destroyed in many places by the cor-rosion process, but it was still possible to uncover the greencorrosion layer on most of the surface. In contrast, the laser-cleaning tests were not successful as the burial deposits couldnot be removed completely. In the areas where the burial de-posits had been removed, most of the green corrosion layerwas removed by the laser and the original surface was lost insome areas down to the metal core. Cottam et al. [17] wholaser cleaned a Roman bronze coin with a transverse excitedatmospheric CO2 laser emitting at 10.6 µm also reported theremoval of a green corrosion layer and loss of surface details.The laser-cleaned surface of Object 4 was matt and had a dis-tinct purple hue. The difference between the surfaces uncov-ered by mechanical cleaning and laser treatment is illustratedin Figure 4. As no liquid had been used while laser clean-ing, a test was made on the back of the object using water.As with Object 1, the results were not significantly differentfrom those obtained when no liquid was used.

Object 5

The green corrosion layer, which contained the original sur-face of Object 5, was retained during mechanical cleaning.In comparison, it was not possible to remove the burial de-posits using the laser without removing the green corrosionlayer, and therefore the original surface was lost. The laser-cleaned surface was mostly covered by cuprite, as identifiedusing Raman spectroscopy and SEM-EDX. The laser-cleanedsurface had a matt and slightly purple appearance.

6 Laser Chemistry

(a)

(b)

Figure 4: Cleaning results for Object 4: (a) laser-cleaned sur-face, (b) mechanically cleaned surface. (Horizontal field of view is5 mm.) Note that the purple hue of the laser-cleaned surface on Ob-ject 4 is not apparent using the microscope.

5. DISCOLOURATION

As reported earlier, there was a slightly purple discoloura-tion to the laser-cleaned surfaces of Objects 1, 2, 4, and 5.For Objects 1 and 2, this was only on some areas, whereason Objects 4 and 5, this was all over the laser-cleaned sur-face. When the green corrosion layer on the mechanicallycleaned surface of Object 5 was lifted using a scalpel, it wasnoticed that the layer underneath the green corrosion layerhad a brown colour. This shows that the unusual colour ofthe laser-cleaned surface has been caused by laser irradiationand was a surface effect. The discoloured areas on Objects2, 4, and 5 were examined using SEM at a high magnifica-tion and it was observed that the purple hue was associatedwith the presence of particles, many of which were spherical.These are shown in Figures 5 and 6. Three spherical parti-cles on Object 4 were analysed using SEM-EDX and foundto contain copper and oxygen. The formation of particles onmetals has been reported in the literature when laser clean-ing tarnished copper coins [13], tarnished silver threads [15]and archaeological iron [14]. It has been suggested that thisis due to the metal vaporising and being redeposited. Theabsence of dicolouration on Object 3 is possibly due to it be-ing the only object with a cerrusite corrosion layer, or due tothe significantly lower fluence at which it was laser cleaned(240 mJ/cm2 compared to 400–900 mJ/cm2 of the other ob-jects). Pini et al. [8] and Batishche et al. [9] did not report anysurface alterations at a microscopic level when cleaning ar-chaeological copper alloys using an Nd:YAG laser at 1064 nm

� 50 µm

Figure 5: SEM photograph showing the presence of particles on thelaser-cleaned surface of Object 5. (Length of scale marker is 50 µm.)

� 20 µm

Figure 6: SEM photograph showing the laser-cleaned surface ofObject 2. (Length of scale marker is 20 µm.)

and µs pulses. The use of longer pulse durations is expectedto increase heat conduction to the bulk material comparedto ns pulses and, as the laser-cleaned surfaces were not ex-amined at high magnification in these studies, surface alter-ations may not have been detected. Alternatively, it is possi-ble that the burial deposits on the objects cleaned in [8, 9]were very different from those examined here and could beremoved at fluences that did not affect the oxidation layer.

6. CONCLUSIONS

From the results of the tests conducted, it can be concludedthat the use of a Q-switched Nd:YAG laser at 1064 nm isnot a suitable method of cleaning archaeological copper al-loy objects. The burial deposits were hard to remove fromthe surface of the objects and the fluences used were dif-ferent for each object probably because of the difference in

C. Korenberg and A. Baldwin 7

composition of burial deposits. On two of the five samples,laser cleaning was not successful at removing these incrusta-tions. With a single laser pulse, it was not possible to controlwhich corrosion layers were removed and which retained.The green corrosion layers containing malachite on Objects4 and 5 were easily removed at the fluences required to cleanoff the burial deposits, exposing the cuprite layer, and on sev-eral objects the metal core was revealed in small areas. Thisresulted in uneven cleaning and loss of detail with the totalremoval of the original surface on those objects where it wascontained within the carbonate corrosion layer. Some of thelaser-cleaned sections had a disfiguring purple hue attributedto the formation of particles that could be seen when examin-ing the surface using SEM. In all examples, the section of eachobject cleaned mechanically using a scalpel-produced supe-rior cleaning results and greater retention of surface detail.Further work will be conducted to explore whether better re-sults can be obtained using lasers with different wavelengthsand pulse durations, as these parameters have been shown toaffect the laser cleaning of metals (e.g., [10, 18]).

ACKNOWLEDGMENTS

The authors would like to acknowledge their colleagueQuanyu Wang for useful discussions and her help with po-larised light microscopy. Thanks must also be given to LeslieWebster and Barry Ager from the Department of Prehistoryand Europe at the British Museum for donating the archae-ological objects for the laser cleaning tests. Capucine Koren-berg thanks the British Academy for awarding her an Over-seas Conference Grant to attend the XI Conference on LaserOptics in St. Petersburg and present the work described inthis paper.

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